1. Field of the Invention
The present invention relates to an internal combustion engine coolant system and more particularly relates to a method and apparatus for transferring heat from and degassing liquid coolant used in such a system. 2. The Prior Art
Internal combustion engine liquid cooling systems have traditionally employed a pump driven by the engine which circulated a liquid coolant through coolant passages in the engine and then through a radiator by which heat was transferred from the coolant to atmospheric air.
The radiators of these coolant systems have generally comprised manifold-like tanks disposed at opposite ends of core tubes. The coolant has been directed to one tank from the engine and thence through the core tubes to the other tank from which the coolant is returned to the engine by the pump. Atmospheric air forced across the core tubes cooled the coolant.
The use of multipass radiators was proposed in some early coolant systems. In these proposed radiators, the coolant traversed the core tubes two or more times before being returned to the engine. These proposed radiators were relatively bulky and complex. Furthermore, continuing improvements in materials generally and in engine and radiator designs over the years resulted in higher engine operating temperatures, pressurized high temperature coolant systems, and more efficient coolant heat rejection. As a result, relatively simple, compact single pass radiators have become standard for use with automotive I.C. engines.
Water has long been the usual coolant for liquid cooled internal combustion engines because of its availability and favorable heat transfer characteristics and the design of engine coolant systems has generally presupposed the use of water as the coolant. In winter, or in arctic locations, antifreeze additives were normally employed to prevent freezing of the water in the systems. These additives were known to alter the heat transfer characteristics of the coolant and reduce the ability of the coolant to reject heat in the radiator, but the normally low temperature of the ambient air compensated for the changes in heat transfer characteristics so that the effectiveness of the coolant system designs was largely unaffected by the use of coolant additives in cold seasons or climates.
Ethylene glycol has been used as a coolant system antifreeze additive for many years. In the past this substance was produced primarily as a coolant additive and therefore was not available in great quantities. Accordingly, ethylene glycol was expensive and its use was substantially limited to seasonal antifreeze protection. Ethylene glycol is now produced as a byproduct of certain processes for making plastics and consequently in recent years the availability of this material has increased markedly while its price has been significantly reduced.
Ethylene glycol has a number of properties which make its permanent use in coolant systems desirable. Engine coolants consisting of a mixture of ethylene glycol and water exhibit a higher boiling temperature than water, retard corrosion in the coolant systems and, as noted, protect the system against freezing particularly due to unpredicted changes in atmospheric air temperature. Accordingly, due to the noted advantages and to the relatively low price of ethylene glycol, coolants composed of ethylene glycol and water have been increasingly used year around in many types and kinds of vehicles.
On the other hand, the specific heat or heat capacity of ethylene glycol-water coolants is less than that of water which restricts the ability of ethylene glycol-water coolants to carry heat away from the engine. Furthermore the film strength of ethylene glycol-water coolants is large when compared to that of water and this characteristic reduces the susceptibility of ethylene glycol-water coolants to deaeration. Consequently, air or gas tends to remain entrained in these coolants which has the effect of reducing their heat transfer capabilities. Year around use of this coolant has thus resulted in reductions in efficiency of the engine coolant systems, particularly in hot weather or warm climates, and as a consequence coolant system capacities have had to be increased. As engine power ratings have been increased over the years the amount of heat required to be dissipated by the radiators has likewise increased. The noted use of ethylene glycol-water coolants has also required that the radiators be modified to increase their abilities to transfer heat from the coolant.
It is well established that the heat transfer characteristics of a given radiator can be increased by increasing the area of the radiator core tubes through which heat is transferred from he engine coolant to atmospheric air. As a result, to increase coolant system capacity the heat transfer areas of the radiators have been increased by increasing the number and/or length of the core tubes. This practice has increased the size and weight of the radiators. Furthermore, the increases in size of the radiators has been disproportionately great when compared to the actual increases in cooling system capacities realized from the larger radiators.
In the passenger car industry, radiator size requirements have tended to conflict with styling considerations and in some circumstances where the conflicting interests of styling and cooling system effectiveness have been compromised, coolant system capacities have been barely adequate. In the trucking industry, the increased radiator sizes have resulted in increased cost, weight and size of tractors and truck cabs.
Heat transfer from the radiators is, as noted above, adversely affected by the presence of entrained gas or air in the coolant. Air is normally entrapped in cooling systems when the systems are filled with coolant. When the engine is operated, this air is entrained in the coolant. Air is also dissolved in the coolants, and when the engine reaches operating temperature the solubility of the air in the coolant is reduced and air comes out of solution resulting in entrained air bubbles in the coolant.
Combustion gases entrained in the coolants are perhaps more common and more troublesome than entrained air. Nearly every engine, particularly after it has been in service for a time, develops combustion gas leaks between the combustion chambers and the engine coolant passages. These leaks are normally such that small quantities of high pressure gases from a combustion chamber will flow into the coolant but the coolant will not normally flow into the combustion chamber because of its low pressure. Accordingly, in most engines combustion gases will flow into the coolant while the engine is operating, become entrained in the coolant, and reduce its heat transfer effectiveness.
Because ethylene glycol-water coolants are characterized by relatively high film strength when compared to water, entrained gas or air is not readily removable from the coolants. In conventional radiator constructions the facilities for deaerating or degassing the coolant in the radiators have not been reliable particularly when ethylene glycol-water coolants are employed. The prior art radiators provided with coolant deaeration constructions have, in the main, presupposed the use of water as an engine coolant. Since the heat transfer properties of water are generally quite favorable, the presence of some entrained air in the water has not been a cause for great concern and deaeration of coolant in radiators has been largely a hit-or-miss proposition. The use of ethylene glycol-water mixtures with their attendent less favorable heat transfer properties and high film strength has increased the need for thorough deaeration while rendering the prior art deaeration techniques even less effective.